Derecho a probar como elemento esencial del debido proceso

A suite of offline and real-time gas- and particle-phase measurements was deployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant and Aerosol Study (SOAS) to examine the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol (SOA) formation. High- and low-time resolution PM2.5 samples were collected for analysis of known tracer compounds in isoprene-derived SOA by gas chromatography/electron ionization- mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography/diode array detection-electrospray ionization-high-resolution quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QToFMS). Source apportionment of the organic aerosol (OA) was determined by positive matrix factorization (PMF) analysis of mass spectrometric data acquired on an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average mass concentrations of the sum of quantified isoprene-derived SOA tracers contributed to ~10% (up to 27%) of the total OA mass, with isoprene-epoxydiol (IEPOX) chemistry accounting for 98% of the quantified tracers. PMF analysis resolved a factor with a profile similar to the IEPOX-OA factor resolved in an Atlanta study and was therefore designated IEPOX-OA. This factor was

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This chapter is in preparation for publication in Atmospheric Chemistry and Physics Discussions.

strongly correlated (r2 > 0.75) with 2-methyltetrols, IEPOX-derived organosulfate, and C5-alkene triols, confirming the role of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracer mass was ~26% (up to 48%) of the IEPOX-OA factor mass, which accounted for 33% of the total OA. A low-volatility oxygenated organic aerosol (LV-OOA) and an oxidized factor with a profile similar to 91Fac observed in areas where emissions are biogenic-dominated were also resolved by PMF analysis, whereas no primary organic aerosol (POA) sources could be resolved. These findings were consistent with low levels of primary pollutants, such as nitric oxide (NO ~0.03 ppb), carbon monoxide (CO ~116 ppb), and black carbon (BC ~0.2 µg m-3). Aerosol sulfate showed a moderate association (r2 = 0.3 – 0.6) with the IEPOX-derived SOA tracers and the IEPOX-OA factor, revealing an influence of anthropogenic emissions on SOA formation from isoprene oxidation. Moderate correlation between the methacrylic acid epoxide (MAE)-derived SOA tracer 2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides (NOy; r2 = 0.38) and nitrate (r2 = 0.45) indicates the potential influence of anthropogenic emissions through long-range transport. Despite the lack of a clear association of IEPOX-OA with locally estimated aerosol acidity and liquid water content, a box model calculations of IEPOX uptake using the simpleGAMMA model, accounting for the role of acidity and aerosol water, predicted the abundance of the IEPOX-derived SOA tracers 2-methyltetrols and the corresponding sulfates with moderate accuracy (r2 = 0.3 – 0.4). Thus, the simpleGAMMA model supports an anthropogenic influence on isoprene-derived SOA formation through acid-catalyzed heterogeneous chemistry of IEPOX due to the consistent presence of acidic sulfate aerosol in the southeastern U.S.

4.2 Introduction

Atmospheric fine particulate matter (PM2.5, aerosol with aerodynamic diameter ≤ 2.5 µm) can scatter and/or absorb solar and terrestrial radiation as well as influence cloud formation and as a result, can markedly affect regional and global climate (IPCC, 2013a). It is now also established that exposure to PM2.5 can have an adverse impact on human health (Dockery et al., 1993, Mauderly and Chow, 2008, Hsu et al., 2011). Organic matter (OM) comprises the largest mass fraction of PM2.5 and is derived largely from secondary organic aerosol (SOA) formed

through atmospheric oxidation of volatile organic compounds (VOCs). SOA formation has been modeled primarily within the framework of absorptive gas-to-particle partitioning (Pankow, 1994, Odum et al., 1996), with the products of volatile and semi-volatile organic precursors decreasing in volatility during multi-generational oxidation, and condensing onto pre-existing particles or creating new particles through nucleation. Recent work has demonstrated the importance of heterogeneous (or particle-phase) chemistry in SOA formation (Jang et al., 2002, Kalberer et al., 2004, Tolocka et al., 2004, Gao et al., 2004, Surratt et al., 2006); however, chemical transport models are only just beginning to incorporate explicit details of this chemistry for specific SOA precursors (Pye et al., 2013, Karambelas et al., 2014). Although much progress has been made in recent years in identifying key biogenic and anthropogenic SOA precursors, significant gaps still remain in our knowledge of the formation mechanisms, composition and properties of SOA (Hallquist et al., 2009).

Isoprene (2-methyl-1,3-butadiene, C5H8) is the most abundant non-methane VOC emitted into Earth’s atmosphere at ~600 Tg yr-1 (Guenther et al., 2006). The southeastern U.S. during summer is a particularly strong source of isoprene, primarily through emissions by broad-leaf trees. Although isoprene is known to influence urban ozone (O3) formation in the southeastern

U.S.,only in the last decade has hydroxyl radical (OH)-initiated oxidation been recognized as leading to significant SOA formation, enhanced by the presence of anthropogenic pollutants such as nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2) (Claeys et al., 2004, Edney et al., 2005, Surratt et al., 2006, Kroll et al., 2006, Surratt et al., 2010). Previously, the volatility of the photochemical oxidation products had been assumed to preclude formation of PM2.5 from isoprene oxidation (Pandis et al., 1991, Kamens et al., 1982).

Recent studies have made significant strides in identifying critical intermediates in isoprene SOA formation by varying the levels of NOx (Kroll et al., 2006, Surratt et al., 2006, Surratt et al., 2010) and acidity of sulfate aerosol (Surratt et al., 2006, Surratt et al., 2010, Lin et al., 2012, Lin et al., 2013a). The proposed role of isomeric isoprene epoxydiols (IEPOX) as key intermediates in the formation of isoprene SOA under low-nitric oxide (NO) conditions (Surratt et al., 2010, Paulot et al., 2009) has recently been confirmed in studies using authentic compounds (Lin et al., 2012, Gaston et al., 2014, Nguyen et al., 2014). Under high-NO conditions, isoprene SOA has been demonstrated to form primarily via oxidation of methacrolein (MACR) (Surratt et al., 2006) and methacryloyl peroxynitrate (MPAN) (Surratt et al., 2010) with methacrylic acid epoxide (MAE) from the further oxidation of MACR demonstrated as a reactive intermediate (Lin et al., 2013b). Under both high- and low-NO conditions, acid-catalyzed reactive uptake and multiphase chemistry of isoprene-derived epoxides (IEPOX and MAE) are now known to enhance SOA formation (Surratt et al., 2007b, Surratt et al., 2010, Lin et al., 2013b). Recent flowtube studies on reactive uptake kinetics of trans-β-IEPOX (Gaston et al., 2014), the predominant IEPOX isomer formed in the photochemical oxidation of isoprene (Bates et al., 2014), have estimated an atmospheric lifetime shorter than 5 h in the presence of highly acidic aqueous aerosol (pH ≤ 1). Since the predicted atmospheric lifetime of IEPOX for gas-

phase oxidation is 8 – 33 hours at an average OH concentration of 106 molecules cm-3 (Jacobs et al., 2013, Bates et al., 2014) and 11 hours for deposition (Eddingsaas et al., 2010), reactive uptake of IEPOX onto highly acidic aqueous aerosol would be a competitive or potentially dominant fate of IEPOX in the atmosphere. Recent field data from sites across the southeastern U.S. collected by Guo et al. (2014) yielded estimates that aerosol pH ranges from 0.5 – 2. Consistent with expectations based on the flowtube studies (Gaston et al., 2014) and pH estimates from field data, IEPOX-derived SOA has been observed to account for up to 33% of the total fine OA mass collected during summer in downtown Atlanta, GA, by analysis of data acquired on an online Aerodyne Aerosol Chemical Speciation Monitor (ACSM) (Budisulistiorini et al., 2013). In offline chemical analysis of total fine OA mass at a rural site located in Yorkville, GA (Lin et al., 2013b) up to 20%, of the OA mass could be attributed to the known IEPOX-derived SOA tracers, including the 2-methylterols (Claeys et al., 2004, Lin et al., 2012), C5-alkene triols (Wang et al., 2005, Lin et al., 2012), cis- and trans-3-methyltetrahydrofuran- 3,4-diols (Lin et al., 2012, Zhang et al., 2012c) and IEPOX-derived organosulfates (Surratt et al., 2007a, Surratt et al., 2010, Lin et al., 2012).

In addition to examining the effects of NO and aerosol acidity on isoprene SOA formation, the effect of varying relative humidity (RH) has recently been examined. In chamber studies on the high-NO pathway under low-RH conditions, the isoprene SOA constituents 2- methylglyceric acids and corresponding oligoesters derived from MACR, MPAN, and MAE, were enhanced relative to higher RH conditions (Zhang et al., 2011, Nguyen et al., 2011). However, 2-methyltetrols, which are known to be major SOA constituents formed in the low-NO pathway and minor constituents in the high-NO pathway (Edney et al., 2005, Surratt et al., 2007b), did not vary significantly with RH (Zhang et al., 2011). While RH appears to have an

effect on the formation of certain isoprene SOA constituents, recent flowtube studies demonstrated that aerosol acidity has a more pronounced effect on IEPOX-derived SOA formation than RH (Gaston et al., 2014). However, field studies have yielded mixed results. At Yorkville, GA, Lin et al. (2013a) observed no strong correlation of IEPOX-derived SOA with aerosol acidity, NH3 levels or liquid water content (LWC), although there was a statistically significant enhancement of IEPOX-derived SOA under high SO2-sampling scenarios. Similarly, no correlation between isoprene SOA tracers and aerosol pH or LWC was observed in the analysis of filter samples collected from field studies in Sacramento, CA, or Carson City, NV (Worton et al., 2013), based on the Extended Aerosol Inorganic Model (E-AIM II) (Clegg et al., 1998). Another recent field study by Budisulistiorini et al. (2013) found weak correlation (r2 = 0.22) between aerosol pH and an IEPOX-OA factor resolved by positive matrix factorization (PMF) from real-time organic aerosol mass spectra data acquired on an Aerodyne ACSM.

Although isoprene is now recognized as a major source of SOA, the exact manner in which isoprene-derived SOA is formed in the southeastern U.S. and how it is affected by anthropogenic pollutants (i.e., NOx level, aerosol acidity, sulfate and primary aerosol loadings) remains unclear. The gap in understanding has major public health and policy implications since isoprene is emitted primarily from terrestrial vegetation and is not controllable, whereas strategies to control anthropogenic pollutants can be implemented. Improving our fundamental understanding of the role of anthropogenic emissions in isoprene SOA formation will be key in improving existing air quality models, especially in the southeastern U.S. where models currently under-predict isoprene SOA formation (Foley et al., 2010, Carlton et al., 2010) and as a result will be critical to developing efficient control strategies for improving air quality. The study presented here is part of the 2013 Southeast Oxidant and Aerosol Study (SOAS) spanning

1 June – 17 July 2013 at the Look Rock (LRK), TN ground site. A major aim of SOAS was to address the issue of how exactly isoprene SOA formation occurs and the potential of anthropogenic emissions to enhance SOA formation. At the LRK ground site we approached this aim by examining the chemical composition of organic aerosol measured in real-time by the Aerodyne ACSM and subsequently applying positive matrix factorization (PMF) for source apportionment. We also collected PM2.5 on filters and quantified tracers associated with isoprene chemistry off line to support the assignment of OA factors resolved from factor analyses of organic mass spectral data collected by the ACSM. We examined the potential influence of anthropogenic emissions on isoprene-derived SOA by correlation with temporal variation of anthropogenic markers monitored by collocated instruments. Finally, a photochemical box model was employed to further examine the potential interactions between SOA and anthropogenic emissions. The results of this study will improve model parameterizations required to bring model predictions closer to ambient observations of isoprene-derived SOA formation in the southeastern U.S.